The invention is related to the field of lubrication of a mechanical device having bodies which are displaceable with respect to one another. Such mechanical device may take several forms; as an example, a rolling element bearing is mentioned. However, the invention is not limited to such device. Other examples include hinged devices and sliding devices.
In said devices, lubrication is applied in order to obtain a smooth behaviour with less friction of the mutually displaceable bodies. Also, it is envisaged to limit wear in this way.
In order to obtain a proper lubrication, it is important to ensure that a thin film of lubrication liquid is formed and maintained in the contact area between the bodies. The formation of such film depends i.a. on the behaviour of the lubrication liquid with respect to pressures, the size and shape of the contact surfaces of the bodies, etcetera.
Once the film is destroyed, e.g. due to exposure to extreme pressures accompanied by a leakage, a direct contact of the bodies is obtained which has a negative influence on the friction and wear.
In the case of a porous material saturated with a viscous liquid, bouncing against the face of a moving body, the interface viscous shear forces deplete the surface region of the porous material quicker than the porous material could replenish the interface. This results in an evacuated surface region that collapses elastically and prevents further flow from the pores of the material. Removal of the load, or the sliding shear, allows the porous interface region to elastically recover and replenish itself with fluid from within the hinterland of the porous material.
The object of the invention is therefore to provide for an improved lubrication, whereby a proper lubricant film can be maintained between two bodies, also under extreme conditions. This object is achieved by means of a lubrication system for mutually displaceable bodies of a mechanical device, said system comprising a porous element and a lubricating liquid comprising dispersed particles, the size of the pores of the porous element, and the size of the particles being selected such that the pores of the porous element become blocked by the particles contained in the lubricant liquid film in the area of contact of said porous element with one of the bodies.
In the lubrication system according to the invention, the porous material represents a reservoir thus allowing to maintain an appreciable amount of lubrication liquid in the vicinity of the contact area between the bodies, which lubricating liquid may escape freely from the pores. On the other hand, the system enhances the formation of a lubricant squeeze film as soon as one of the bodies approaches or bounces into the porous material. This is caused by the fact that the pores in the porous element become temporarily blocked by the particles dispersed in the lubricant. Said lubricant then can no longer escape via the pores, and forms a squeeze film in the contact area of those bodies.
The use of particles as a secondary phase in a base lubricant, will enhance the possibility of maintaining a film under squeeze (high pressure gradient) conditions.
In particular, platelet like particles can be applied which have a maximum diameter engineered to be larger than the bearing""s pore size, and which will align themselves in the film flow direction, but will tend to obstruct the flow of lubricant into the pores under squeeze load induced pressure gradients (film pressure greater than pore pressure). When the pressure gradient is reversed the particles would tend to leave the surface and allow the porous bearing surface to release lubricant into the film.
Furthermore, the porous material can be engineered such that its effective pore size increases toward the film boundary. In that connection, a particle size can be employed that will enter the pores at this boundary, but will not pass through the porous media under a loading pressure gradient. Under a reversed pressure gradient, reverse flow will aid the release of particles back into the film.
Having regard to the properties of elastic porous media, as described earlier, and the viscous forces in the film which deplete the near surface layers under thin-film high shear-rate conditions, thus causing elastic deformation/collapse, the effective pore size is reduced such that secondary phase particles are trapped in the surface region rendering the surface layers impervious. When loads are removed, the reverse pressure gradient and the elastic recovery of the surface layers/region allow the particles and lubricant to pass back into the film.
In general, a sufficiently low modulus of elasticity is required to allow deformation of the passages for the operating pressure gradient.
In essence, particles will be engineered to match film thickness conditions and bearing material pore size and structure, to exploit the combination, in a form of micro non-return valve principle, that can enhance lubrication under dynamic load conditions.
Such lubrication system resembles the human spherical joints (e.g. knee) where the long proteins in the synovial fluid block the fibrous channels into the cartilage, thus creating a squeeze film during the approach of the contacting surfaces as the joint gets loaded (e.g. standing). Upon release the channels get xe2x80x9cunpluggedxe2x80x9d, and lubricating fluid is released.
Analogously, once the bodies in question move away from one another, the pores are freed and oil is subsequently released.
The size of the particles may be larger than the size of the pores. Moreover, the porous material should be wettable by the lubricating liquid.
Preferably, the lubrication liquid is an oil comprising PTFE particles.
According to a particular preferred embodiment of the system according to the invention, one of the bodies is a cage for a rolling element bearing, said cage having pockets defined by a porous material, and the other body is a rolling element of said bearing and contained in such pocket.
In the case of rolling element bearings there is also a significant potential further advantage offered from using the porous cage as a lubricant supply to the raceway contacts. Such contacts, particularly at high speeds, often run partially starved due to adverse geometry and the continuous passage of rolling elements expelling lubricant from the tracks, and there not being sufficient time to replenish them with new lubricant prior to the next overolling encounter. Rolling elements continually oscillate in the cage pockets and thus provide the dynamic load environment that is needed to exploit the porous material as a possible lubricant source, and at an ideal place to satisfy the raceway contact needs.
Two additional areas where benefits are envisaged are; big-end bearings, particularly in large marine engines where inertial forces are relatively high; and the cage-sphere interfaces in constant velocity ball joints. Improvements in the lubrication of such devices would offer significant advantages in terms of reducing frictional losses and improving efficiency, that could lead to higher performance ratings and energy savings.